Within-Population Developmental and Morphological Plasticity is Mirrored in Between-Population Differences: Linking Plasticity and Diversity

A review of the empirical evidence for costs of plasticity in ectothermic animals

Phenotypic plasticity can represent a vital adaptive response to environmental stressors, including those associated with climate change. Despite its evolutionary advantages, the expression of plasticity varies significantly within and among species, and is likely to be influenced by local environmental conditions. This variability in plasticity has important implications for evolutionary biology and conservation physiology. Theoretical models suggest that plasticity might incur intrinsic fitness costs, although the empirical evidence is inconsistent and there is ambiguity in the term 'cost of plasticity'. Here, we systematically review the literature to investigate the prevalence of costs associated with phenotypic plasticity in ectothermic animals. We categorized studies into those assessing 'costs of phenotype' (trade-offs between different plastic trait values) and 'costs of plasticity' (intrinsic costs of the capacity for plasticity). Importantly, the experimental designs required to detect costs of plasticity are inherently more complex and onerous than those required to detect costs of phenotype. Accordingly, our findings reveal a significant focus on costs of phenotype over costs of plasticity, with the former more frequently detecting costs. Contrary to theoretical expectations, our analysis suggests that costs of plasticity are neither universal nor widespread. This raises questions about the evolutionary dynamics of plasticity, particularly in stable environments. Our analysis underscores the need for precise terminology and methodology in researching costs of plasticity, to avoid conflating costs associated with plastic traits with costs more intrinsic to plasticity. Understanding these nuances is crucial for predicting how species might adapt to rapidly changing environments.

Subtle environmental variation affects phenotypic differentiation of shallow divergent treefrog lineages in Amazonia

Amazonia harbours a vast biotic and ecological diversity, enabling investigation of the effects of microevolutionary processes and environmental variation on species diversification. Integrative approaches combining phenotypic and genetic variation can improve our knowledge on diversification processes in megadiverse regions. Here, we investigate the influence of environmental and geographic variation on the genetic and morphological differentiation in the Amazonian Boana calcarata-fasciata (Anura: Hylidae) species complex. We analysed the variation of one mtDNA gene from individuals of different forest environments, and assessed their phylogenetic relationships and species limits to define the lineages to perform a phenotypic-environmental approach. We collected morphological data (head shape and size) using 3D models and investigated the phylogenetic signal, evolutionary model and influence of environmental variables on morphology. We verified associations between environmental and geographical distances with morphological and genetic variation using distance-based redundancy analyses and Mantel tests. We found an even higher cryptic diversity than already recognized within the species complex. Body size and head shape varied among specimens, but did not present phylogenetic signal, diverging under a selective evolutionary model. Our results show that diverse factors have influenced morphological and genetic variation, but environmental conditions such as vegetation cover, precipitation and climate change velocity influenced morphological diversification. Possible population-level mechanisms such as parallel morphological evolution or plastic responses to similar environments could account for such patterns in these typical Amazonian treefrogs.

Geographical differences in competitive hierarchy in a native–invasive system

Biological invasions can create novel competitive interactions and force ecological shifts in both native and invasive species. Anuran tadpoles are able to modify their behaviour, morphology, growth and development to cope with competitive pressure. This plasticity is a good target for natural selection and can drive rapid evolutionary changes in response to novel interactions. Here, we explore changes in plastic responses and fitness of competing invasive and native tadpoles by exposing tadpoles from different locations with contrasting evolutionary histories to the same set of varied competitive conditions. Eggs were collected from one site near the first introduction of the invasive frog (~110 years of coexistence) and from a second site that was invaded recently. We hypothesized less favourable outcomes for the invasive species in long-coexisting populations, where the native competitor might have developed adaptive responses. Most results support the hypothesis. Where the invasion was older, invasive tadpoles exposed to native competitors grew less, developed more slowly and displayed morphologies linked to competitive stress, whereas the developmental stability and canalization of native tadpoles increased. On the whole, the asymmetric competitive relationship thus appeared to approach symmetry after ~35 generations, highlighting a noteworthy example of rapid adaptation after an invasion.

De Novo Assembly and Annotation of the Larval Transcriptome of Two Spadefoot Toads Widely Divergent in Developmental Rate

Amphibians are highly vulnerable and diverse vertebrates for which we still have modest genomic resources. Amphibian larvae are key components of continental wetlands, where they have strong influences on energy fluxes, nutrient cycling, and community structure. Amphibian larvae are highly sensitive to environmental conditions and can often alter their physiology, behavior and even morphology in response to the local conditions experienced, although we still know relatively little about the transcriptomic changes that enable such plasticity. Here we contribute the larval transcriptomes of two spadefoot toad species with divergent developmental rates and degree of developmental plasticity in response to pond drying.

Morphological homeostasis in the fossil record

Morphological homeostasis limits the extent to which genetic and/or environmental variation is translated into phenotypic variation, providing generation-to-generation fitness advantage under a stabilizing selection regime. Depending on its lability, morphological homeostasis might also have a longer-term impact on evolution by restricting the variation-and thus the response to directional selection-of a trait. The fossil record offers an inviting opportunity to investigate whether and how morphological homeostasis constrained trait evolution in lineages or clades on long timescales (thousands to millions of years) that are not accessible to neontological studies. Fossils can also reveal insight into the nature of primitive developmental systems that might not be predictable from the study of modern organisms. The ability to study morphological homeostasis in fossils is strongly limited by taphonomic processes that can destroy, blur, or distort the original biological signal: genetic data are unavailable; phenotypic data can be modified by tectonic or compaction-related deformation; time-averaging limits temporal resolution; and environmental variation is hard to study and impossible to control. As a result of these processes, neither allelic sensitivity (and thus genetic canalization) nor macroenvironmental sensitivity (and thus environmental canalization) can be unambiguously assessed in the fossil record. However, homeorhesis-robustness against microenvironmental variation (developmental noise)-can be assessed in ancient developmental systems by measuring the level of fluctuating asymmetry (FA) in a nominally symmetric trait. This requires the analysis of multiple, minimally time-averaged samples of exquisite preservational quality. Studies of FA in fossils stand to make valuable contributions to our understanding of the deep-time significance of homeorhesis. Few empirical studies have been conducted to date, and future paleontological research focusing on how homeorhesis relates to evolutionary rate (including stasis), species survivorship, and purported macroevolutionary trends in evolvability would reap high reward.

Geographical variations in adult body size and reproductive life history traits in an invasive anuran, Discoglossus pictus

Variability in life history traits positively affects the establishment and expansive potential of invasive species. In the present study, we analysed the variation of body size in seven populations - two native and five invasive - of the painted frog (Discoglossus pictus, Anura: Discoglossidae), native to North Africa and introduced in southern France and the north-east of the Iberian Peninsula. Other life history traits (age at maturity, size at maturity, longevity, median age and potential reproductive lifespan) were analysed in a native and an invasive population. We observed geographic variations in adult body size, related mainly to mean annual precipitation. Thus, populations had greater body size as mean annual precipitation increased, resulting in bigger specimens in the invasive populations. Adult body size and growth rates also varied between sexes in all studied populations, with males significantly larger than females. Age distribution varied between native (1-5 years) and invasive populations (2-4 years) and also between sexes. Our results suggest that higher precipitation promotes faster growth rates and larger adult body size that could facilitate the successful establishment of invasive populations.